(1) Detailed equilibrium and kinetic studies have suggested a mechanism by which multivalent immune complexes interact with Fc receptors. The mechanism emphasizes the role of receptor density on the affinity with which a cell binds multivalent ligands. (2) A fluorescent assay employing flow microfluorometry has been developed for quantitating Fc receptors on individual cells. The method is especially useful for studying heterogeneous populations of cells. (3) IgG Fc receptors were studied and compared with the IgE receptors on rat basophilic leukemia cells. IgG and IgE were bound by distinct receptors; the IgE receptor binds only IgE, while the IgG receptor binds both IgG and IgE. (4) P388D1 cells were found to internalize immune complexes rapidly at 37 degrees. Internalization also occurs for IgG-dimers, trimers, and perhaps monomers. Subsequent to internalization, IgG is degraded to a 50,000 M.W. component. (5) Effector and target cells have been studied in "franked" ADCC assays. It has been found that mouse and rat spleen cells and human peripheral blood lymphocytes can serve as effectors in this system. Tumor cells from a human B-cell line can be lysed by franked rat and human cells.